This invention concerns improvements in the field of non-destructive testing and failure analysis using pulsed ultrasound as a material probe. While many of the applications of the invention are useful in a broad range of applications, the invention will be described in the context of acoustic micro imaging (“AMI”). To further an understanding of the broad applicability of the principles of the invention, a brief description will be engaged of various scanning modes commonly employed today in AMI.
The most basic form of acoustic interrogation is illustrated in FIG. 1 in which an ultrasonic transducer 20 is excited with a sharp electrical pulse and emits a pulse of ultrasonic energy which is brought to a focus 22 within sample 24 by a lens (not shown) at the distal end of the transducer 20. FIG. 1A is intended to represent the sample 24 as having a front surface A, an internal interface (acoustic impedance mismatch), and a bottom surface C. An acoustic (sometimes termed herein “ultrasound”) pulse is reflected from the front surface A, interior interface B and bottom surface C and sensed by the transducer 20.
The amplitude of the reflected acoustic waveform as a function of time is shown in highly simplistic form FIG. 1B. The waveform 26 is commonly known as the “A” waveform or “A-scan”, and in practice contains a great deal of information about acoustic impedance perturbations or features in the body of the sample.
In waveform 26 the first spike 28 is the “main bang” resulting from the electrical excitation of the transducer 20. A second spike 30 occurs later in time and is the sensed reflection from front surface A of the sample body 24. Still later in time, the transducer senses a reflection 32 from the interface B, and finally a reflection 34 from the rear surface C of the sample body 24.
As will be described in more detail below, acoustic microscopes employ a time window or “gate” 36 which passes only returned reflections which lie within a certain span of time corresponding to a certain depth in the sample. In FIG. 1B gate 36 is set to pass only the signal representing interface B reflection 32.
FIG. 2 represents in highly schematic form the “C-Mode” scan wherein a focused image is formed of an X-Y plane at a specific depth in the Z axis. FIG. 2A is another illustration of a C-Mode scan, showing the A-scan waveform produced from an interrogation by transducer 42 of three different Points 1, 2, and 3 in the sample. At Point 1, the A waveform 44 shows a reflection 46 at the front surface and a smaller reflection from an interface 48 in the sample 50. A waveform 52 associated with X-Y Point 2 shows a reflection 54 from an air gap 56 formed in the interface 58. The reflection 54 shows a phase reversal because of the lower impedance of air than of the sample material. Waveform 59 associated with Point 3 reveals a small amplitude reflection 60 from what may be an occlusion 62 in the body of the sample 50. The polarity of the reflection 60, being the same as that of the first reflection 63, suggests that the reflection 60 is not from an air void or other feature having lower acoustic impedance than that of the sample material.
In C-Mode scanning, a gate is set, as shown at 43 in FIG. 2A, for example, within which a peak detector (not shown) detects the peak value of the gated signal segments. The peak values detected are stored in a 2D (spatial) X-Y memory (not shown).
In C-Mode scanning acoustic microscopy a focused spot of ultrasound is generated by an acoustic lens assembly at frequencies typically in the range of 10 MHz to 200 MHz or more. The ultrasound is conducted to the sample by a coupling medium, usually water or an inert fluid. The angle of the rays from the lens is generally kept small so that the incident ultrasound does not exceed the critical angle of refraction between the fluid coupling and the solid sample. The focal distance into the sample is shortened by the refraction at the interface between the fluid coupling and the solid sample.
The transducer alternately acts as sender and receiver, being electronically switched between transmit and receive modes. A very short acoustic pulse enters the sample, and return acoustic reflectances are produced at the sample surface and at specific impedance interfaces and other features within the sample. The return times are a function of the distance from the encountered impedance feature to the transducer and the velocity of sound in the sample material(s).
An oscilloscope display of the acoustic reflectance pattern (the A scan) will clearly show the depth levels of impedance features and their respective time-distance relationships from the sample surface.
This provides a basis for investigating anomalies at specific levels within a part. The gated acoustic reflectance amplitude is used to modulate a CRT that is one-to-one correlated with the transducer position to display reflectance information at a specific level in the sample corresponding to the position of the chosen gate in time.
With regard to the depth zone within a sample that is accessible by C-scan techniques, it is well known that the large acoustic reflectance from a liquid/solid interface (the top surface of the sample) masks the small acoustic reflectances that may occur near the surface within the solid material. This characteristic is known as the dead zone, and its size is usually of the order of a few wavelengths of sound.
Far below the surface, the maximum depth of penetration is determined by a number of factors, including the attenuation losses in the sample and the geometric refraction of the acoustic rays which shorten the lens focus in the solid material. Therefore, depending upon the depth of interest within a sample, a proper transducer and lens must be used for optimum results.
In C-Mode scanning acoustic microscopy (“C-SAM”), contrast changes compared to the background constitute the important information. Voids, cracks, disbonds, and other impedance features provide high contrast and are easily distinguished from the background. Combined with the ability to gate and focus at specific levels, C-SAM is a powerful tool for analyzing the nature of any acoustic impedance feature within a sample.
In this type of C-mode scanning, the A-scan for each point interrogated by the ultrasonic probe is discarded except for the image value(s) desired for that pixel. Two examples of image value data are: (a) the peak detected amplitude and polarity, or (b) the time interval from the sample's surface echo to an internal echo (the so-called “time-of-flight” of “TOF” data).
FIG. 3 illustrates the “B-Scan” mode which produces nondestructive cross-section data displayed as amplitude values of digital samples of A-Scan waveforms arrayed in a simulated X-Z plane of the sample.
FIG. 4 is another mode related to the B-Scan mode and is termed by the assignee of this invention as the Quantitative B-Scan Analysis Mode, or “Q-BAM”. A Q-BAM scan produces a calibrated, nondestructive cross-section of data in the X-Z plane of the sample. The data captured is caused to be completely in focus through the entire Z depth by scanning at various Z positions and readjusting the transducer focus before each successive scan. The position of the probe focus and the gate are automatically linked such that the gated segment of the waveform always represents reflections from impedance features which are in focus.
FIG. 5 illustrates a scan mode known as Three Dimensional Time-of-Flight or “3D TOF” which internally tracks first interface topography within a sample. Color-coded 3D graphic imagery is commonly employed to show the TOF topography of the inspected feature in relation to its distance from the top surface of the inspected sample.
FIG. 6 shows a transmission mode which investigates the entire thickness of the sample in one scan. It is the ideal scan mode for rapidly identifying gross anomalies such as a die disbond. The anomaly detected can later be isolated and inspected in detail with C-Mode analysis.
FIG. 7 represents a combination of reflection and transmission mode scanning. In one X-Y scan the entire thickness and a specific interface or anomaly can be inspected.
As represented schematically in FIG. 8, AMI is commonly employed to automatically position, focus, scan, analyze and report on acoustically detectable features in a tray of parts such as integrated circuits. FIG. 9 shows a similar application for ultrasonically inspecting various parts or locations on a PC board. The process is designed to automatically examine multiple types of parts located at specific locations on the PC board.
FIG. 10 depicts a “bulk-scan” mode which provides two dimensional (X,Y) display and measurement of material properties throughout a predetermined gated thickness (Z depth) in the examined part.
Multi-Scan (FIG. 11) is a way to obtain multiple C-Mode images with one scan at preseleted interfaces, or obtain a C-Mode and Bulk-Scan image simultaneously. It is ideally suited for applications such as simultaneous overmold material and interface bond analysis.
FIG. 12 illustrates an “R-Scan” mode—a rotational scanner which locates hidden defects within the circumference of a cylindrical sample. Nondestructively it “unwraps” and displays two dimensional (X,theta) image from 0 to 360 degrees of rotation.
FIG. 13 is a mode for scanning spherical surfaces and subsurfaces. Hidden defects and flaws are first located and then C-Mode imaged for confirmation.
FIG. 14 a screen print of a monitor image formed using a commercial C-Mode scanning acoustic microscope manufactured by the assignee of the present invention. The sample was an encapsulated integrated circuit having a die attached to a pad, with leads extending radially from all sides of the die.
As noted, the “time of flight” of the acoustic pulses from transducer to sample front surface was 20.50 microseconds. The front end (“FE”) gain was set at 22.500 dB, and the slice gain at 26.000 dB. The gate was positioned at 0.736 microseconds from the front surface echo and had a width of 0.564 microseconds in order to capture a depth in the package embracing the die leads and the die-pad interface. The transducer frequency was 15 MHz and the transducer focal length was 0.774 inch.
Acoustic reflectance signals 66, 68, 70 were stimulated by the transducer at three location, numbered “1”, “2”, and “3”, respectively. Location “1” was on a bonded lead. The white color in the image reproduction signifies a sound bond between the inspected lead and the encapsulating material. Corresponding acoustic reflectance signal 66 shows a reflection 72 from the front surface of the package. Less than 1 microsecond later, we see a positive polarity reflection 74 from the soundly bonded lead. As the reflection 74 is within the reproduction gate 76, the reflection 74 is rendered in the image 64.
However, with the probe at position “2” over a different lead, we see in acoustic reflectance signal 68 a negative polarity reflection 80, indicating that the acoustic wave encountered an interface with a lower acoustic impedance than that of the sample material. The logical interpretation of this data is that the lead at position “2” is disbanded and that the resulting air gap is responsible for the phase reversal of the reflection 80. Again, because the reflection 80 is within the gate, it is visualized in the image 64.
With the probe at position “3” on the die-pad interface, acoustic reflectance signal 70 shows a negative polarity reflection from the interface, indicating a die-pad disbond (air gap). The location of the reflection 82 closer to the front surface reflection 72 indicates that the die-pad interface is slightly higher (closer to the probe) than the leads at positions “1” and “2”.
As described above, in conventional C-Mode AMI, the only data that is captured and stored for display and analysis is the peak value of the amplitude waveform within the gate 76, optionally along with polarity and TOF data. All other data are lost. Note in each of the three acoustic reflectance signals 66, 68, 70, the acoustic impedance perturbations appearing as ripples 84, 86, 88 at depths below the gate 76. Also, the ripples 85, 87, 89 occurring above the gate 76,78. Using conventional C-Mode AMI, the potentially valuable information contained in those ripples is lost forever.
Thus, from the above description it is well known in the field of acoustic image microscopy to capture an X-Y set of data points, each point representing the detected peak of a gated region of an amplitude-modulated acoustic signal reflected from impedance features within the body of an insonified solid part. The set of data points may be visualized, for example on the screen of a computer monitor.
By moving the position of the gate along the time axis of the acoustic reflectance signal, a particular plane or layer within the examined part may be inspected. To increase the resolution capability of the X-Y dataset at varying depths through the solid, it is known to coincide the focus of the acoustic transducer employed with the gated region of the part.
Referring to FIG. 15, this process “slices” the part into as many horizontal sections as desired. Typically 10 slices are adequate for thin samples such as integrated circuits, however, up to 200 slices can be made on commercially available AMI equipment. Equipment software can be set to divide the part into equal thickness slices. Each slice is then automatically scanned with the focus and gate optimized for each specific slice depth. As in C-Mode scanning, the reflectance signals are gated and peak amplitude values are stored. After scanning, the slices may be displayed simultaneously as “thumbnail” images on a computer monitor screen. Typically the slices are reviewed in detail or thumbnail images are reviewed in a slide show sequence—a process analogous to descending acoustically through the part or peeling it apart layer by layer.
The described equipment has software which assembles the slices and reconstructs the data into an “acoustic solid”. The acoustic solid can be rotated to any desired angle of view. With this software, an operator can also visualize cross-sections of the acoustic solid. Sectioning can take many different forms—a single horizontal, vertical or diagonal section can be removed. Multiple sections or “bits” can be removed which are defined by material properties, rather than by geometry. For example, an operator might remove all of the molding compound from an IC package and still leave intact the image of a crack within the molding compound.
As described, the acoustic solids created by effectively stacking an array of such X-Y data sets have proven to be useful in certain applications where the information desired to be recorded and displayed is simply the signal peaks in the gated regions of interest. However, many applications exist wherein the information desired to be retrieved from the solid part is not precisely known at the time the part is acoustically interrogated. Since the acoustic reflectance signals returned from each pulse are lost, except for the peak of the signal within the gated region (and optionally, polarity and TOF data), it is not possible to derive additional information from the stored X-Y-Z data set of amplitude peak values. It is not possible off-line, for example, to adjust the position of the gate to change the depth of the inspected plane. Nor is it possible to employ signal processing analytics on the missing acoustic signal information to learn more about the anomalies which perturbed the insonifying sound pulses.
Another prior art approach employed in AMI differs from the above description in at least respect. One X-Y scan across a sample is employed with a transducer having a high F-number lens. The acoustic reflectance signal (A-scan) is captured, digitized, and stored as digital data. Because the capture gate may be as wide as the specimen (or the zone of interest) is deep, reflections from all levels in the specimen are stored. After capture of the ungated, full volume waveform, a gate (or multiple gates) can be introduced at any position to visualize reflecting features in a specimen at a depth and thickness corresponding to the gate location and width.
In this approach, the acoustic waveform for each pixel is saved to storage means (memory, hard drive, etc.). The number of samples in a stored waveform may vary depending on the application and any implementation specific limits. For example, 1 to 4 microseconds of the acoustic waveform, including and after the surface echo, may be stored. At a 1 GHz sampling rate, this would represent 1000 to 4000 samples per each pixel's acoustic waveform. The total number of pixels in an image depends on the chosen image resolution. Typical image resolutions include: 256×240, 512×480, and 1024×960. The total storage required grows rapidly as the resolution increases.
The last-described scan mode provides the data for a 2-dimensional B-scan. Depending upon the transducer, either all or only a small part of the stored waveform may be within the focused region of the ultrasonic beam. Or, depending upon the transducer and the sample, either all or only a small part of the sample's thickness may be covered by the stored waveform. Or, depending upon the transducer and the sample, either all or only a small part of the sample's thickness may be within the focused region of the ultrasonic beam.
The advantage of this approach is that the full acoustic reflectance waveform is captured and digitized for later review, gating and processing. No information is irretrievably discarded as in conventional C-Scan approach which employs gated peak detection and storage of gated signal peaks (which become the image pixels upon display), and, optionally polarity and time-of-flight data. As described, to produce a display of any particular level or levels within the sample, the gating is set for that level or levels.
One obvious and significant disadvantage of this approach, however, is that all images generated by changing gate position are not in sharp focus. An attempt is made to place the entire specimen depth in focus which means that no plane is in the sharpest possible focus. As will become evident from the ensuing description, another drawback of moment of this last-described approach is the limited amount of data that can be gathered in a single scan of the sample.